The invention relates to a submersible motor for driving a centrifugal pump.
A typical submersible motor is for example shown and described in EP-A-0 346 730. It drives a multi-stage centrifugal pump and together with this, forms a submersible pump unit. The submersible motor is of the canned motor construction type, in whose canned pot the motor is fastened on a common shaft on which the impeller group of the centrifugal pump is mounted. The rotor space is sealed by an end plate which carries the pump-side radial bearing of the motor, for supporting the common shaft, and by a sealing formation opposite the pump suction space. The sealing formation comprises a contact seal resting on the shaft, for example a lip seal and volume compensation means, for example a diaphragm, for the fluid located in the rotor space. This fluid which as a rule is a glycol-water mixture and which serves for removing the heat from the motor as well as serving as a lubrication means for the motor bearings, circulates in a closed circuit which consists of the rotor space of the motor and the hollow space of the common shaft. Such submersible pump units are commonly driven in deep wells and are often driven at high rotational speed.
Since a dynamic sealing (contact sealing) does not absolutely seal, in the course of the operating period of the submersible pump unit, an exchange of fluid from the rotor space with a component of the delivery flow must be taken into account. This loss of fluid from the rotor space is then not disadvantageous when, for this, delivery fluid reaches into the rotor space. If however fluid with gases dissolved therein are delivered and then components thereof seep into the rotor space during which the rotor space fluid is displaced, problems arise which increase the more the unit is driven in the part load region.
Although the previously described submersible pump unit has generally proved itself, those problems with the delivery of fluids which have a relatively high gaseous component and which are delivered with a high rotational speed lead to bearing damage on the submersible motor which leads to a premature stoppage of the submersible pump unit. The seeping in of a considerable quantity of gas, released from the delivery flow in the pump suction space and pushed to the sealing location on the motor shaft, into the rotor space of the motor under displacement of fluid from the rotor space, seems to be the cause of this.